The present invention is directed to resistance balancing, and more particularly to a mass flow sensor that is capable of detecting the mass flow rate of a fluid by balancing the resistance of upstream and downstream temperature sensors.
Mass flow sensors are used in a wide variety of applications to measure the mass flow rate of a gas or other fluid. One application in which a mass flow sensor may be used is a mass flow controller. In a conventional mass flow controller, the mass flow rate of a fluid flowing in a main fluid flow path is regulated or controlled based upon a mass flow rate of a portion of the fluid that is diverted into a typically smaller conduit forming a part of the mass flow sensor. Assuming laminar flow in both the main flow path and the conduit of the sensor, the mass flow rate of the fluid flowing in the main flow path can be determined (and regulated or controlled) based upon the mass flow rate of the fluid flowing through the conduit of the sensor.
Two different types of mass flow sensors have traditionally been used, constant current mass flow sensors, and constant temperature mass flow sensors. An example of a constant current mass flow sensor is illustrated in FIG. 1. In FIG. 1, a fluid flows in a sensor pipe or conduit in the direction of the arrow X. Heating resistors or xe2x80x9ccoilsxe2x80x9d R1 and R2 having a large thermal coefficient of resistance are disposed about the sensor conduit on downstream and upstream portions of the sensor conduit, respectively, and are provided with a constant current I from a constant current source 901. As a result of the constant current I flowing through the coils R1 and R2, voltages V1 and V2 are developed across the coils. The difference between voltages V1 and V2 (V1xe2x88x92V2) is taken out of a differential amplifier 902, with the output of the amplifier 902 being proportional to the flow rate of the fluid through the sensor conduit.
At a zero flow rate, the circuit of FIG. 1 is configured so that the resistance value (and thus, the temperature) of coil R1 is equal to the resistance value (and temperature) of coil R2, and the output of the amplifier 902 is zero. As fluid flows in the sensor conduit, heat that is generated by coil R2 and imparted to the fluid is carried towards R1. As a result of this fluid flow, the temperature of coil R2 decreases and that of coil R1 increases. As the voltage dropped across each of these resistors is proportional to their temperature, voltage V1 increases with an increased rate of fluid flow and voltage V2 decreases, with the difference in voltage being proportional to the mass rate of flow of the fluid through the sensor conduit.
An advantage of a constant current mass flow sensor is that it can operate over a wide range of temperatures, is relatively simple in construction, and is responsive to changes in the ambient temperature of the fluid entering the sensor conduit. In this regard, as the ambient temperature of the fluid entering the sensor conduit changes, so does the resistance of each of the coils R1 and R2. However, it takes a relatively long time for the temperature (and thus, the resistance) of the coils R1 and R2 to stabilize in response to a change in the rate of flow of the fluid.
The other type of mass flow sensor that is frequently used is a constant temperature mass flow sensor, examples of which are illustrated in FIGS. 2-4. As shown in the constant temperature mass flow sensor of FIG. 2, heating resistors or xe2x80x9ccoilsxe2x80x9d R1a and R1b are respectively disposed about the downstream and upstream portions of a sensor conduit through which a fluid flows in the direction of the arrow X. As in the constant current mass flow sensor of FIG. 1, each of the downstream and upstream coils R1a and R1b has a large thermal coefficient of resistance. The resistance (and thus the temperature) of each of the coils R1a, R1b is fixed by separate and independent circuits to the same predetermined value that is governed by the value of resistors R2a, R3a, R4a, and R2b, R3b, R4b, respectively. Control circuitry is provided to maintain each of the coils R1a, R1b at the same predetermined value of resistance (and thus, temperature) independently of the rate of fluid flow through the sensor conduit.
In the absence of fluid flow, the circuit of FIG. 2 is configured so that the resistance (and temperature) of each of the downstream and upstream coils R1a and R1b is set to the same predetermined value and the output of the circuit is zero. As fluid flows in the sensor conduit, heat from the upstream coil R1b is carried towards R1a. As a result, less energy is required to maintain the downstream coil R1a at the fixed temperature than is required to maintain the upstream coil R1b at that same fixed temperature. The difference in energy required to maintain each of the coils R1a, R1b at the predetermined temperature is measured and is proportional to the mass flow rate of fluid flowing through the sensor conduit.
The constant temperature mass flow sensor described with respect FIG. 2 is also relatively easy to construct. In addition, the circuit of FIG. 2 stabilizes more quickly in response to changes in the mass flow rate of the fluid entering the sensor conduit than the constant current mass flow sensor described with respect to FIG. 1. However, because each of the coils R1a and R1b is set and maintained at a predetermined temperature independently of the ambient temperature of the fluid flowing into the sensor conduit, a problem arises when the ambient temperature of the fluid flowing into the sensor conduit increases. In particular, when the ambient temperature of the fluid flowing in the sensor conduit approaches the predetermined temperature that is maintained by the upstream and downstream coils, the circuit loses its ability to discern differences in the flow rate of the fluid, and when the ambient temperature of the fluid increases beyond this predetermined temperature, the sensor is rendered inoperable.
To overcome these disadvantages, a number of alternative constant temperature mass flow sensors have been provided. For example, the circuit of FIG. 3 provides a constant temperature mass flow sensor that is capable of responding to changes in the ambient temperature of a gas or fluid, at least to a certain degree. Once again, R1b and R2b are downstream and upstream temperature sensing coils with a large temperature coefficient of resistance. However, rather than maintaining the temperature of the coils at a predetermined constant value as in the circuit of FIG. 2, the circuit of FIG. 3 maintains the temperature of the sensor coils R1b, R2b at a temperature that is above the ambient temperature of the fluid flowing into the sensor conduit. This is achieved by the insertion of an additional coil R3b, R4b having a coefficient of resistance similar to that of the sensor coils R1b, R2b in each of the downstream and upstream circuits. As the ambient temperature of the fluid changes, the series addition of coil resistance R3b, R4b to the temperature setting resistors R5b, R6b results in raising the temperature to which the upstream and downstream resistance coils are maintained above the ambient temperature of the fluid flowing into the sensor conduit. As in the circuit of FIG. 2, the difference in energy supplied by each of the downstream and upstream circuits to maintain the temperature of the coils R1b, R2b at the same temperature is proportional to the mass flow rate of the fluid through the sensor conduit.
As should be appreciated by those skilled in the art, for the circuit of FIG. 3 to operate properly, it is critical that the values and thermal characteristics of each element in the downstream circuit match that of the corresponding element in the upstream circuit. Thus, the resistance of the downstream and upstream coils R1b, R2b must have the same value, and the same thermal coefficient of resistance. In addition, resistor R3b must have the same value and the same (ideally large) thermal coefficient of resistance as resistor R4b, resistor R5b must have the same value and same (ideally zero) thermal coefficient of resistance as resistor R6b, resistor R7b must have the same value and same (ideally zero) thermal coefficient of resistance as resistor R10b, resistor R9b must have the same value and same (ideally zero) thermal coefficient of resistance as resistor R8b, and amplifiers 911 and 912 must have the same operating and temperature characteristics.
Despite the addition of resistors R3b and R4b, a problem with the circuit of FIG. 3 is that as the ambient temperature of the fluid flowing into the sensor conduit rises, the sensor becomes less accurate because the proportional difference between the temperature of the upstream and downstream coils relative to the temperature of the ambient fluid becomes smaller. Further, there is a problem due to drift in that the calibration of the sensor at one temperature does not necessarily allow its use at other ambient temperatures without some sort of compensation circuit.
To solve some of the aforementioned problems, U.S. Pat. No. 5,401,912 proposes a constant temperature rise (above ambient) mass flow sensor, an example of which is shown in FIG. 4. The circuit of FIG. 4 acts to maintain upstream and downstream sensor coils R2, R1 at a predetermined value above the ambient temperature of the fluid flowing into the sensor conduit. The circuit of FIG. 4 is identical to the circuit of FIG. 2, except that the fixed value resistors R3a and R3b of FIG. 2, which have an essentially zero thermal coefficient of resistance, are replaced with resistors R5 and R6, respectively, having a large and specific valued thermal coefficient of resistance. As a result of these changes, the circuit of FIG. 4 purportedly maintains a constant temperature rise over the ambient temperature of the fluid flowing into the sensor conduit. Such a mass flow sensor as is shown in FIG. 4 is therefore termed a constant temperature difference (over ambient) or a constant temperature rise (over ambient) mass flow sensor.
Each of the aforementioned constant temperature mass flow rate sensors utilizes separate and independent upstream and downstream circuits to set the temperature of the upstream and downstream coils to a particular value, or to a particular value over the ambient temperature of the fluid flowing into the sensor conduit. A disadvantage of each of these circuits is that they require a close matching of corresponding circuit elements (i.e., resistors, coils, and amplifiers) in the upstream and downstream circuits.
According to one embodiment of the present invention, a sensor is provided that includes a first resistor, a second resistor, a first circuit, and a second circuit. The first and second resistors each has a resistance that varies in response to a change in a physical property. The first circuit is electrically coupled to the first resistor and sets the resistance of the first resistor. The second circuit is electrically coupled to the second resistor and adjusts the resistance of the second resistor to equal the resistance of the first resistor. A processing circuit may be coupled to the first and second circuits to measure a difference in an amount of energy provided by the first and second circuits to the first and second resistors, respectively. Where the resistance of the first and second resistors varies in response to a change in temperature, and the first and second resistors are disposed about a conduit in which fluid flows, the sensor is capable of measuring a mass flow rate of the fluid flowing in the conduit.
According to another embodiment of the present invention, a mass flow sensor is provided. The sensor includes a first heat sensitive coil, a second heat sensitive coil, a first circuit, a second circuit, and a processing circuit. The first and second heat sensitive coils are disposed at spaced apart positions about a conduit through which a fluid flows, and each has a resistance that varies with temperature. The first circuit is electrically coupled to the first heat sensitive coil and sets the resistance of the first heat sensitive coil to a value that corresponds to a predetermined temperature. The second circuit is electrically coupled to the second heat sensitive coil and adjusts an amount of current provided to the second heat sensitive coil so that the resistance of the second heat sensitive coil equals the resistance of the first heat sensitive coil. The processing circuit is coupled to the first and second circuits and measures a difference in an amount of energy provided by the first and second circuits to the first and second heat sensitive coils, respectively.
According to a further embodiment of the present invention, a method of balancing a resistance of a first resistor and a resistance of a second resistor is provided. The resistance of the first and second resistors vary with temperature, and the method includes acts of setting the resistance of the first resistor to a first value and providing an amount of current to the second resistor so that the resistance of the second resistor matches the first value of the first resistor.
According to another embodiment of the present invention, a method of setting the resistance of a resistor is provided. The method includes acts of: (a) measuring an ambient temperature of a fluid flowing into a conduit about which the resistor is disposed, (b) incrementing the ambient temperature measured in act (a) by a predetermined amount to identify a temperature to which the resistor is to be set, (c) calculating a value of resistance corresponding the temperature identified in act (b), (d) determining a division ratio to be provided by a programmable voltage divider to force the resistance of the resistor to the value calculated in act (c), and (e) configuring the programmable voltage divider to provide the division ratio determined in act (d).